/Engine/Source/Runtime/SlateCore/Public/FastUpdate/SlateInvalidationWidgetSortOrder.h
SlateInvalidationWidgetIndex cannot be used to sort the widget since the ArrayIndex may not be in the expected order. (See the array as a double linked list). SlateInvalidationWidgetSortOrder builds a unique number that represents the order of the widget. The number is padded in a way to keep the order but not necessarily sequential. It is valid until the next SlateInvalidationRoot::ProcessInvalidation()
struct FSlateInvalidationWidgetSortOrder